Who are the people on U.S. paper currency?

I've always assumed that the figures printed on the bills are American presidents. But thinking more carefully, Benjamin Franklin on the $100 bill isn't actually a president.
U.S. law doesn't require that presidents be depicted on the currency; it only sets a basic criterion that the person must be deceased. The specific choice of who appears is decided by the Secretary of the Treasury.
Just took a quick look at the figures on the dollar bills for fun.